How to check legality for free content libraries?

Question

Copyright

Good afternoon.
There are many libraries of free content on the web for their projects (music, textures, etc.), mostly under Creative Commons licenses or similar.
As I understand it, it is very difficult to make sure that the content is legal. For example, someone posted someone else's work on their behalf.
Who is responsible and what are the consequences of using such content in your project?
In theory, a similar situation is if, under the contract, the freelancer did not create content to order, but stole it.
Thank you.